Heretofore, various proposals have been made for an ultrasonic motor of a type linearly driving a driven element. Such an ultrasonic motor is disclosed, for example, in Japanese Laid-open Patent Publication No. 2004-304887. The driving principle of this motor is described below with reference to FIGS. 17, 18A, and 18B.
FIG. 17 is a perspective view showing the appearance of a linear-type ultrasonic motor.
As shown in FIG. 17, the linear-type ultrasonic motor 510 is comprised of a vibrator 501 and a linear slider 506. The vibrator 501 includes a piezoelectric element 505 formed into a rectangular thin plate, and a vibrating body 502 integrally joined to one surface of the piezoelectric element 505. The vibrating body 502 has a rectangular-shaped base and two protruding portions 503-1, 503-2 projecting from an upper surface of the base.
FIGS. 18A and 18B are views showing the forms of two vibration modes (MODE-A and MODE-B) excited in the vibrator of FIG. 17.
Each of the two vibration modes is a bending vibration mode excited in the out-of-plane direction of the plate-shaped vibrator 501. The vibrator 501 is designed such as to resonate in these modes at frequencies approximately equal to each other.
Two lower parts of FIG. 18A show the vibrator 501 as seen from the Y direction. When the MODE-A vibration is excited in the vibrator 501, there are formed three vibration nodes α as shown in the lowest part of FIG. 18A (a second-order bending vibration mode). Each vibration node extends in the Y direction of the vibrator 501
As shown in FIG. 18A, the rectangular column-shaped contact members 503-1, 503-2 are disposed in the vicinity of the nodes formed by the MODE-A vibration. The MODE-A vibration causes contact surfaces 504-1, 504-2 of the contact members 503-1, 503-2 to reciprocally move in the X direction, as shown by an arrow.
Two lower parts of FIG. 18B show the vibrator 501 as seen from the X direction. When the MODE-B vibration is excited in the vibrator 501, two vibration nodes β are formed (a first-order bending vibration mode), as shown in the lowest part of FIG. 18B. Each vibration node extends in the X direction of the vibrator 501. Thus, the nodes generated in the MODE-A vibration and the nodes generated in the MODE-B vibration extend perpendicular to each other in the XY plane.
As shown in FIG. 18B, the contact members 503-1, 503-2 are disposed in the vicinity of the antinode formed by the MODE-B vibration. The MODE-B vibration causes a reciprocal motion of the contact surfaces 504-1, 504-2 in the Z direction.
The above-described vibration modes are excited in the vibrator 501 by an inverse piezoelectric effect caused when a desired AC signal is input to the piezoelectric element 505. When the excitation is such that a phase difference between the MODE-A vibration and the MODE-B vibration is nearly equal to +π/2 or −π/2, near elliptic motions of the contact surfaces 504-1, 504-2 are generated in the XZ plane in FIG. 17. By virtue of the elliptic motions, a relative displacement motion occurs between the vibrator 501 and the linear slider 506 disposed in pressure contact with the contact surfaces 504-1, 504-2.
Next, an explanation is given of a distortion generated in the vibrator 501 when the above described vibration modes are excited in the vibrator 501.
With reference to FIG. 18A, the case where the MODE-A vibration is generated is first described. Positive and negative signs (+), (−) in FIG. 18A each represent the direction of an X directional component of distortion observed when the vibrator 501 is deformed by the vibration (Ditto in FIG. 18B). The positive sign (+) represents that the piezoelectric element 505 is elongated in the X direction, and the negative sign (−) represents that the piezoelectric element 505 is contracted in the X direction.
The sings (+), (−) indicates that each half of the piezoelectric element 505 is divided into two regions in the thickness direction in terms of the direction of distortion. At the boundary between the two regions, there is a plane where the X directional distortion is not produced, which is referred to as the neutral plane T1. It is also understood that the sign (distortion direction) is reversed between both halves of the piezoelectric element 505 with respect to the X-direction center portion thereof (FIG. 18A).
In the case where the MODE-B vibration is generated, it is understood that the piezoelectric element 505 is divided into two regions in the thickness direction thereof in terms of the signs (+), (−) each representing the direction of the Y directional distortion of the piezoelectric element 505 (FIG. 18B). The boundary between the two regions is referred to as the neutral plane T2.
It is generally known that smooth contact can be achieved between a linear slider and contact members disposed in pressure contact therewith in an ultrasonic motor when each contact member has a spring function.
Nevertheless, in the prior art ultrasonic motor, both the linear slider 506 and the two contact members 503-1, 503-2 provided integrally with the vibrating body 502 are each made of an inorganic material that has no spring function in its structure, and therefore, smooth contact cannot be achieved.
The present inventors designed a contact member 609 having a spring function, which is shown in FIG. 19. The contact member 609 is comprised of a protruding portion 603, two fixing portions 607 that support the protruding portion 603, and spring portions 608. The protruding portion 603 is required to be formed into a protrusion having a contact surface 604 thereof disposed in contact with a linear slider (not shown), which is driven in the X direction in FIG. 19. Therefore, the protruding portion 603 includes shoulder portions 613 respectively extending from two long side edges of the contact surface 604 of a rectangular shape.
The two fixing portions 607 are disposed in a direction parallel to the moving direction of the linear slider at a distance therefrom. The vibration body 602 is formed with a groove 612 having a sufficient depth such that the spring portions 608, horizontally extending from the fixing portions 607, can each have a spring function.
With the introduction of the above described construction, however, the following new problem is exposed.
As described above, the signs (+), (−) respectively represent the elongation and contraction of the piezoelectric element in the X direction in the MODE-A of FIG. 18A. By virtue of the elongation and contraction in the X direction, the two fixing portions 607 of the contact member 609 are moved toward and away from each other in the X direction.
FIGS. 20A to 20C are views, as seen from the Y direction in FIG. 19, showing how the contact member 609 is deformed when the fixing portions 607 in FIG. 19 are elongated and contracted in the in-plane direction.
FIG. 20A shows a shape of the contact member 609 observed when no vibration is generated in the vibrator 601, FIG. 20B shows a shape of the contact member 609 observed when the fixing portions 607 are moved away from each other, and FIG. 20C show a shape of the contact member 609 observed when the fixing portions 607 are moved toward each other. Larger arrows each represent the direction of displacement of the contact surface 604. Due to deformation of the shoulder portions 613, the contact surface 604 is displaced in the Z direction (the vertical direction in FIGS. 20A to 20C) in synchronization with movements of the fixing portions 607. At the moment when the displacement of FIG. 18A is generated, the right-side protruding portion is located at the crest of vibration, and thus the contact member 609 disposed thereat is deformed as shown in FIG. 20B. On the other hand, since the left-side protruding portion is located at the trough of vibration, the contact member 609 disposed thereat is deformed as shown in FIG. 20C. Accordingly, in the arrangement where the two contact members 609 are used instead of the contact members 503-1, 503-2 of the vibrator of FIGS. 18A and 18B, the contact members 609 are displaced in the Z direction in antiphase with each other.
By making the phase difference between the reciprocal motion in the x direction in the MODE-A and the reciprocal motion in the Z direction in the MODE-B to be nearly equal to +π/2 or −π/2, near elliptic motion of the contact surface 604 is generated. In the arrangement using the contact member 609, however, in the MODE-A, the contact surface 604 makes a reciprocal motion that results from the combination of the displacement in the X direction and the displacement in the Z direction.
Specifically, as seen from the Y direction of the coordinate system of FIG. 19, a trajectory of FIG. 21B is obtained in the above described arrangement, which is different from a trajectory in the prior art example shown in FIG. 21A. Furthermore, in the MODE-A, two projections are displaced in the Z direction in antiphase with each other.
Therefore, if the trajectory of the contact surface 604 of one of the two contact members 609 is as shown in FIG. 21B, then the trajectory of another contact surface 604 becomes as shown in FIG. 21C.
For the above reasons, in the arrangement using the contact members each having a spring function, the direction of the reciprocal motion in the MODE-A is largely deviated from the horizontal direction, and as a result, the elliptic trajectory is distorted, thereby posing problems such that stable contact cannot be attained and unusual noise is generated, in addition to problems such as increase of sliding loss and acceleration of the progress of wear.